This invention relates to a method for forming corrugated paperboard and more particularly to a method and apparatus for forming corrugated paperboard at a high rate of speed and of high quality.
Systems currently in use for forming corrugated paperboard employ fluted corrugating rolls for corrugating a medium, generally paper drawn off of a roll, to form a corrugated layer. In order to facilitate corrugation and to ensure that corrugations are formed and retain their shape, the paper is generally first conditioned by the application of heat and one or more steam showers applied upstream of the corrugating rolls. The application of heat and steam is believed to "plasticize" the paper fibers to enhance flute formation in the corrugating rolls.
The conditioned corrugating medium is then passed through the corrugating rolls, which are generally heated to temperatures above 340.degree. F., adhesive is applied to the crests of the corrugated paper flutes and paper linerboard or facing is applied to form a single faced corrugated paperboard. If desired, the single faced corrugated paperboard is then double faced in order to enhance the strength of the corrugated product. A second paper linerboard or facing is drawn over a preheating roller and joined with the exposed crests of the flutes of the single faced corrugated medium to which adhesive has been applied. The double faced corrugated paperboard is then passed over a heated surface or hot plate section while subjected to pressure from a weight roller and belt assembly above the hot plate section to cure the adhesive.
One problem in a conventional corrugating system is the occurrence of malformed flutes in the corrugated medium. When the corrugating medium is passed through the corrugating rolls, the corrugations are formed as the medium is pressed in the nip of the corrugating rolls under the application of heat and pressure to form flutes A low flute occurs when the flute does not fully form in the nip of the corrugating rolls. These malformed flutes have crests of uneven amplitudes which produce areas of little or no contact between the linerboard applied to the crests of the corrugated medium in subsequent double facing operations, thus forming a flawed corrugated product.
Another problem occurs when the flute are deformed in the removal of the corrugated medium from the corrugating rolls. Deformation during removal of the corrugated medium from the corrugating rolls results primarily from the inability of the corrugated medium to properly release from the corrugating rolls after the application of heat and pressure in the nip of the rolls. Such deformation substantially reduces the strength of the corrugation and results in a low quality product.
The paper available in today's market is frequently recycled paper and it is expected that more recycled material will be supplied in the future. This recycled paper retains some undesirable additives and/or contaminants and is also not always uniform in fiber content. The result is a paper medium which is difficult to corrugate using existing methods without greatly reducing the rate for producing the corrugated paperboard.
Malformed or deformed flutes result in a reduction in the strength of the corrugated product. In today's market, corrugated products are generally manufactured to conform to various industry and government specifications. The loss in product quality resulting from such malformed and deformed flutes results in rejected goods, customer complaints, and returned shipments. One way in which both of the above problems have been minimized according to the prior art is to run the corrugating system at a low rate of production. This low rate of production is, however, significantly below the speeds at which the corrugating machines are capable of operating and is basically an unsatisfactory method for solving the problem. Obiously, a higher rate of production results in a more cost competitive product.
The prior art has generally attempted to solve the flute malformation problem by preconditioning the corrugating medium with steam, using both a heated roll filled with steam with slots or outlets to release the steam across the width of the paper and a top steam shower. The fibers of the paper become softened and pliable as a result of this steam treatment. The steam, however, is applied some distance (several feet in most cases) before the paper enters the corrugating rolls and is therefore not completely effective at the nip of the corrugating rolls where the corrugations are formed.
In addition, the prior art has attempted to solve the flute deformation problem by the use of various "lubricants" applied to the corrugating medium or to the corrugating rolls. As shown in U.S. Pat. Nos. 1,796,542, 3,676,247, and 3,103,459, the "lubricants" chosen are generally of a hydrocarbon base such as paraffin, wax, or polyethylene. Such hydrocarbon base lubricants have been applied in liquid form or have been formed as a solid wax in solid bar form often being applied to the paper stock or to the crests of the corrugating rolls. Although these lubricants probably do reduce the coefficient of friction between the paper and the corrugating rolls, it is now believed that these materials actually act to further condition the paper by improving the flexibility of the paper fibers and produce some improvement in flute formation and higher operating speeds. However, the use of such hydrocarbon-base materials results in additional problems. Such materials have a tendency to be absorbed by the paper or linerboard with a resulting discoloration of the paper or linerboard. The hydrocarbon materials also tend to vaporize in an oily smoke under the operating temperatures of the corrugating rolls and the hot plates. This oil vapor can result in unacceptable concentrations of hydrocarbons in the plant atmosphere.
U.S. Pat. No. 3,676,247 further discloses a lubricant containing stearins for use in an unheated or low heat corrugating system but the wax base provided for the lubricant would be unacceptable in a heated system for the reasons discussed above. This reference discloses the use of stearins only in a low temperature system (below 320.degree. F.) and only in combination with a hydrocarbon carrier. It will also be noted from the references that the lubrication is applied to either the corrugating medium itself or to the crests of the flutes of the corrugating rolls. The lubrication, when applied to the corrugating rolls, is thus only a topical application and is not evenly distributed over the surface of the flutes of the corrugating rolls.
One prior art reference, U.S. Pat. No. b 3,849,224 to Hintz, discloses the application of water and water with a detergent directly to the corrugating rolls to act as a lubricant. However, Hintz teaches only the corrugation of a plastic coated paper at temperatures of about 2l0.degree. F. where water is a known lubricant for plastic and the addition of a detergent acts as a surfactant to improve these lubricating properties. The water applied by Hintz does not, and can not, act to steam condition the plastic coated paper corrugating medium to reduce the occurrence of malformed flutes. Hintz has no application to the production of corrugated paperboard, where any application of water per se to the paperboard can produce extreme quality variations unless the water is carefully applied in limited amounts in specific areas of the process.
It should be noted that the paper and paperboard have been treated with various products such as metallic stearates to impart a finish or waterproofing to such paper products as typically shown in U.S. Pat. No. 2,029,390. Large quantities of the metallic stearates are required to obtain these altered paperboard characteristics and such applications have no bearing on the production of normal corrugated paperboard. Metallic stearates have also been used as release agents in metal casting and plastic molding operations. Such agents have not heretofore been applied in the paper corrugating industry to improve corrugating system performance.
The disadvantages of the prior art are overcome in the present invention, however, and improved methods and apparatus are provided for forming a single faced corrugated paperboard at increased production rates and with improved flute formation and reduced equipment operating costs.